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fix(openai-transport): handle Mistral reasoning_content as array/object in completions stream (#67192)
Mistral's OpenAI-compatible streaming endpoint returns reasoning_content as an array of block objects rather than a flat string: {delta: {reasoning_content: [{type: "thinking", content: "..."}]}} The existing getCompletionsReasoningDelta() only checked typeof value === "string", so the array/object fell through unhandled. When the value was later used in a template literal or concatenation, JavaScript coerced it to "[object Object]", corrupting the stream and crashing the channel with repeated [object Object] output. Fix: extend getCompletionsReasoningDelta() to handle three formats for the reasoning_content / reasoning / reasoning_text fields: 1. string (existing, unchanged) 2. array of block objects: [{type:"thinking",content:"..."} | {type:"text",text:"..."} | string] 3. single plain object: {type:"thinking",content:"..."} or {type:"text",text:"..."} Adds 4 tests in openai-transport-stream.test.ts covering array blocks, plain object, string backward compat, and empty array (no crash).
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‎src/agents/openai-transport-stream.test.ts‎

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@@ -2018,3 +2018,224 @@ describe("openai transport stream", () => {
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]);
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});
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});
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describe("Mistral reasoning_content object/array handling (#67192)", () => {
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// Shared model fixture for Mistral completions
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const mistralModel = {
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id: "mistral-small-latest",
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provider: "mistral",
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api: "openai-completions" as const,
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baseUrl: "https://api.mistral.ai/v1",
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reasoning: true,
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input: ["text" as const],
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cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
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contextWindow: 128000,
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maxTokens: 16384,
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};
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function makeOutput() {
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return {
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role: "assistant" as const,
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content: [],
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api: mistralModel.api,
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provider: mistralModel.provider,
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model: mistralModel.id,
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usage: {
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input: 0,
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output: 0,
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cacheRead: 0,
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cacheWrite: 0,
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totalTokens: 0,
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cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
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},
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stopReason: "stop",
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timestamp: Date.now(),
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};
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}
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it("extracts reasoning from reasoning_content array of block objects", async () => {
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const output = makeOutput();
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const stream: { push(event: unknown): void } = { push() {} };
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const mockChunks = [
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{
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id: "cmpl-mistral-array",
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object: "chat.completion.chunk" as const,
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choices: [
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{
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index: 0,
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delta: {
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reasoning_content: [
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{ type: "thinking", content: "Let me think about this." },
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{ type: "thinking", content: " More reasoning." },
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],
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} as Record<string, unknown>,
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logprobs: null,
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finish_reason: null,
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},
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],
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},
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{
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id: "cmpl-mistral-text",
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object: "chat.completion.chunk" as const,
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choices: [
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{
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index: 0,
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delta: { content: "Here is the answer." },
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logprobs: null,
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finish_reason: "stop",
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},
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],
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},
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];
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async function* mockStream() {
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for (const chunk of mockChunks) {
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yield chunk as never;
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}
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}
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await __testing.processOpenAICompletionsStream(mockStream(), output, mistralModel, stream);
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expect(output.content).toMatchObject([
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{ type: "thinking", thinking: "Let me think about this. More reasoning." },
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{ type: "text", text: "Here is the answer." },
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]);
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});
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it("extracts reasoning from reasoning_content plain object (single block)", async () => {
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const output = makeOutput();
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const stream: { push(event: unknown): void } = { push() {} };
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const mockChunks = [
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{
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id: "cmpl-mistral-obj",
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object: "chat.completion.chunk" as const,
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choices: [
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{
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index: 0,
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delta: {
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reasoning_content: { type: "thinking", content: "Single block reasoning." },
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} as Record<string, unknown>,
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logprobs: null,
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finish_reason: null,
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},
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],
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},
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{
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id: "cmpl-mistral-text",
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object: "chat.completion.chunk" as const,
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choices: [
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{
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index: 0,
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delta: { content: "Answer." },
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logprobs: null,
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finish_reason: "stop",
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},
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],
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},
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];
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async function* mockStream() {
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for (const chunk of mockChunks) {
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yield chunk as never;
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}
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}
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await __testing.processOpenAICompletionsStream(mockStream(), output, mistralModel, stream);
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expect(output.content).toMatchObject([
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{ type: "thinking", thinking: "Single block reasoning." },
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{ type: "text", text: "Answer." },
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]);
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});
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it("still handles reasoning_content as a plain string (backward compat)", async () => {
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const output = makeOutput();
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const stream: { push(event: unknown): void } = { push() {} };
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const mockChunks = [
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{
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id: "cmpl-mistral-str",
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object: "chat.completion.chunk" as const,
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choices: [
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{
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index: 0,
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delta: { reasoning_content: "String reasoning." } as Record<string, unknown>,
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logprobs: null,
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finish_reason: null,
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},
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],
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},
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{
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id: "cmpl-mistral-end",
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object: "chat.completion.chunk" as const,
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choices: [
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{
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index: 0,
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delta: { content: "Done." },
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logprobs: null,
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finish_reason: "stop",
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},
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],
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},
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];
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async function* mockStream() {
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for (const chunk of mockChunks) {
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yield chunk as never;
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}
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}
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await __testing.processOpenAICompletionsStream(mockStream(), output, mistralModel, stream);
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expect(output.content).toMatchObject([
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{ type: "thinking", thinking: "String reasoning." },
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{ type: "text", text: "Done." },
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]);
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});
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it("ignores empty reasoning_content arrays without crashing", async () => {
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const output = makeOutput();
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const stream: { push(event: unknown): void } = { push() {} };
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const mockChunks = [
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{
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id: "cmpl-mistral-empty",
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object: "chat.completion.chunk" as const,
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choices: [
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{
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index: 0,
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delta: { reasoning_content: [] } as Record<string, unknown>,
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logprobs: null,
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finish_reason: null,
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},
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],
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},
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{
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id: "cmpl-mistral-end",
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object: "chat.completion.chunk" as const,
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choices: [
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{
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index: 0,
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delta: { content: "Just text, no reasoning." },
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logprobs: null,
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finish_reason: "stop",
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},
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],
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},
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];
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async function* mockStream() {
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for (const chunk of mockChunks) {
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yield chunk as never;
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}
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}
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await __testing.processOpenAICompletionsStream(mockStream(), output, mistralModel, stream);
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// Should have only text, no thinking block
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expect(output.content).toMatchObject([{ type: "text", text: "Just text, no reasoning." }]);
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expect(output.content.some((b: { type: string }) => b.type === "thinking")).toBe(false);
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});
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});

‎src/agents/openai-transport-stream.ts‎

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@@ -1218,6 +1218,43 @@ function getCompletionsReasoningDelta(delta: Record<string, unknown>): {
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if (typeof value === "string" && value.length > 0) {
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return { signature: field, text: value };
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}
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// Mistral returns reasoning_content as an array of block objects:
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// [{type: "thinking", content: "..."}, ...]
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// or occasionally as a plain object {type: "thinking", content: "..."}
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// rather than a flat string. Without this branch the value coerces to
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// "[object Object]" when used in a template literal, corrupting the stream
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// and crashing the channel. (#67192)
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if (Array.isArray(value)) {
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let text = "";
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for (const item of value) {
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if (item && typeof item === "object") {
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const block = item as Record<string, unknown>;
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// Mistral block shapes: {type:"thinking",content:"..."} or {type:"text",text:"..."}
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if (typeof block.content === "string" && block.content.length > 0) {
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text += block.content;
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} else if (typeof block.text === "string" && block.text.length > 0) {
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text += block.text;
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}
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} else if (typeof item === "string" && item.length > 0) {
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text += item;
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}
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}
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if (text.length > 0) {
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return { signature: field, text };
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}
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} else if (value !== null && value !== undefined && typeof value === "object") {
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// Plain object block: {type: "thinking", content: "..."}
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const block = value as Record<string, unknown>;
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const text =
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typeof block.content === "string"
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? block.content
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: typeof block.text === "string"
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? block.text
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: undefined;
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if (text && text.length > 0) {
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return { signature: field, text };
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}
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}
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}
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return null;
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}

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